In 2015 the Global Rust Reference Center (GRRC) detected two new races of yellow rust in Scandinavia and in countries around the Baltic Sea.

Yellow rust is a fungal disease which can cause epidemics in wheat and triticale. The spread of fungal spores by the wind over large distances is just one of the challenges in relation to efficient prevention and control of yellow rust. Another challenge is the ability of the fungus to quickly develop new races, which may change crop susceptibility to rust from one growth season to the next.

Timely surveillance of new races is therefore a crucial part of yellow rust early warning in Denmark and the rest of the world. New DNA genotyping methods have been developed that provide rapid and more precise information about individual fungal races, their relationship and origin.

Fungal virulence and aggressiveness

The virulence of yellow rust races defines which crop varieties are at risk, whereas aggressiveness is defined by the ability to cause disease in susceptible varieties. Aggressiveness can be measured by the number of days or hours between transmission and release of the next generation of spores. The recent incursions of aggressive yellow rust from the Himalayas to Europe has therefore increased the need for timely and efficient early warning systems in Europe and elsewhere.

In the future, characterization of virulence and aggressiveness will continue to be based on well-established epidemiological techniques. However, new DNA-genotyping methods may provide information about the relationship and origin of the individual strains.

Advanced genetic methods

One of the advantages of the SSR microsatellite technique is that it can be applied to DNA from infected leaves, which allows the determination of genotype within a few days. SCAR is another PCR-based technique that is easily applied and used to detect specific genotypes. New SCAR markers developed at GRRC may be used to detect specific aggressive yellow rust races. These races are now predominant in major parts of North America, Australia, the Middle East and Africa, where they often cause severe epidemics.

- The new DNA techniques contribute to an increased understanding of the dynamics and distribution of the fungus, thus allowing plant breeders, extension and farmers to be as prepared as soon as possible when new races appear. However, DNA techniques cannot replace race testing and assessment of aggressiveness, which provides direct information about infections in actual wheat and triticale varieties, says postdoc Tine Thach from the Department of Agroecology at Aarhus University, where the Global Rust Reference Center is located at AU Flakkebjerg

Replacement of the past yellow rust population in Europe

Since 2011 the European yellow rust population has been largely replaced by races from the centre of diversity in the Himalayan region. These new races, which had never been detected in Europe, appeared aggressive and were characterized by black “winter spores” (telia) emerging from early on in the growing season. Wheat and triticale varieties that used to be resistant now became susceptible or partly susceptible whereas other varieties became less susceptible.

High genetic variation was demonstrated already in the first year, which was clearly different from the European population before 2011. The wide geographical distribution of the new races and DNA genotyping showed that they were likely carried by wind, resembling the characteristics of yellow rust populations in the centre of diversity in the Himalayas (see the article “Invasions out of centre of diversity increase the risk of disease epidemics in wheat”)

Two new races detected in 2015

The most recent results from GRRC revealed two new races in 2015 in Scandinavia and in countries around the Baltic Sea. The race termed ‘Triticale2015’, which is clearly of non-European origin, has not been reported before from anywhere in the world. Preliminary results indicate that the new ‘Triticale2015’ race is highly aggressive in many triticale varieties but only in a few winter wheat varieties. The origin of the other new race, termed ‘Hereford’ after the wheat variety where it was first detected, is not yet resolved.

The implications of the new races for the coming season are currently being investigated in experiments in the quarantine green house, where commercial varieties and advanced breeding lines of wheat and triticale are tested. Results are expected in February-March 2016. New inoculation methodologies are being applied in adult plant tests focusing on varieties of relevance for organic farming.

Results from race typing of isolates of both European and non-European origin have been updated 2 February 2016. Click here to see more.

The research is funded by the Green Development and Demonstration Programme (GUDP) of the Ministry of Environment and Food of Denmark, Innovation Fund Denmark, the Swedish Board of Agriculture, Bill & Melinda Gates Foundation, USA, and Department for International Development, United Kingdom.